We are developing a method of measuring nuclear estrogen receptor (Rn) and receptor fragments independent of radioactive ligand binding, using a set of monoclonal antibodies against receptor determinants. We will then study the fate (processing) of Rn after translocation by estrogen vs. antiestrogen, we hope to distinguish between several proposed possibilities for the fate of Rn. a) RN is not lost from the nucleus but merely changed into a new form which extracts less easily with salt and is less easily measured by exchange assay techniques. b) Rn is lost from nuclei but in addition estradiol is converted to estrone which binds to a new protein which accumulates in the cytoplasm. c) Rn is lost from nuclei following a dephosphorylation step which prevents all ligand binding. d) Rn binding to antiestrogen results in an additional 55,000 MW protein association.